The present invention generally relates to motor drivers for electrical motors and more particular to apparatus and methods of dissipating regenerative inductive currents in motor drives.
A schematic drawing of a prior art motor driver 10 for an electrical motor 12 (the ProAct Generation 1, which is a form of Limit Angle Torque motor (LAT) commercially available from Woodward Governor) is illustrated in FIG. 1. The motor driver 10 serves the purposes of conditioning the electrical power received from an external power source 14 and maintaining the proper electrical power levels for driving the electrical motor 12. The motor driver 10 includes a diode 16 for ensuring one reverse voltage protection, an electromagnetic (EMI) filter 17 for filtering out high frequency interference, and a bus capacitor 18 for storing electrical power and smoothing out any spikes or intermittent declines in the electrical power and a switch network for modulating electrical energy to the motor.
In the prior art circuit illustrated in FIG. 1, it should first be noted that the motor driver 10 is not integrated with electrical motor 12 but instead is intentionally mounted remotely such that the motor driver 10 is subject to relatively low temperatures of about a maximum of 85xc2x0 Celsius. The motor 12 is driven by an H-bridge configuration comprising four switches 20, 21, 22, 24. When the first and fourth switches 20, 23 are closed (with switches 21, 22 open), the motor 12 is driven in a first rotational direction. When the second and third switches 21, 22 are closed (with switches 20, 23 open), the motor 12 is driven in a second rotational direction.
During normal steady state operation, the net flow of current is out of the motor driver 10 and into the motor 12. However, when it is desired to switch the direction of flow quickly and therefore switch the motor direction, the electrical motor 12 momentarily acts as a generator and forces inductive xe2x80x9cflybackxe2x80x9d current into the motor driver 10. If there were no line impedance 26, EMI filter 17 or diode 16, the inductive flyback current could be dissipated in the external power source 14 assuming that the power source 14 is capable of withstanding the amount of inductive energy produced by the motor 12. However, in prior art designs, the diode 16 has not allowed the electrical flow to reverse and is necessary to prevent destruction of the motor driver in the event that the battery is installed the opposite way. The way the prior art has dealt with this specific problem has been to incorporate an aluminum electrolytic capacitor 18 with a large capacitance of 3000 xcexcF to handle and temporarily store this inductive flyback current load. The aluminum electrolytic capacitor 18 has allowed for a very high capacitance to volume ratio along with a low cost to capacitance ratio.
It is an objective of the present invention to provide an electrical motor that may include integral electronics including the motor driver, which can operate at an elevated temperature of over 100xc2x0 Celsius (typical approaching about 115xc2x0 Celcius) at which temperature aluminum electrolytic capacitors are not reliable.
It is a further objective in regard to the previous objective to achieve the foregoing object without creating cost or size drawbacks.
In accordance with these and other objectives, the present invention is directed toward a novel motor driver with an active snubber circuit. The motor driver is interposed between an external electrical power source and an electrical motor. The electrical motor is driven in first and second states such that the electrical motor produces an inductive flyback current when it switches between states. The motor driver comprises a reverse voltage protector in series between the electrical power source and the electrical motor allowing flow of electrical power to the electrical motor; and a bus capacitor arranged in parallel circuit with the motor (or switch network of the motor driver) between electrical motor and the reverse voltage protector. The active snubber circuit is arranged in parallel circuit with the bus capacitor (or switch network) between the reverse voltage protector and the electrical motor. The active snubber circuit comprises a switch having a circuit that senses an increase in voltage on the bus when inductive flyback occurs. A resistor is in series with the switch. When the voltage of the bus increases, the resistor is switched into the circuit and diverts the inductive flyback current. The snubber circuit dissipates inductive flyback current through the resistor.